Japanese Laid-open Patent No. 62-132060 discloses a control method for a transmission by which a clutch of the transmission is smoothly engaged. The configuration of the proposed clutch hydraulic control device is shown in a sectional view in FIG. 8 and in a hydraulic circuit diagram in FIG. 9. In FIG. 9, a clutch hydraulic control device 50 operates a clutch hydraulic control valve 52 in response to a command from a controller 51, and sends pressurized oil from a hydraulic pump 53 to a clutch chamber 54 to achieve a smooth engagement. The clutch hydraulic control valve 52 includes valves for a forward gear 52A, a reverse gear 52B, a first speed gear, a second speed gear, and the like.
The clutch hydraulic control valve 52 includes a pressure control valve 60 for controlling the clutch oil pressure, a flow rate detecting valve 70 for detecting the pressurized oil flowing from the hydraulic pump 53 to the clutch chamber 54 via the pressure control valve 60 and the flow rate detecting valve 70, and a filling completion detecting sensor 80 (hereinafter called a filling sensor 80) for detecting that the clutch chamber 54 is filled with oil, that is, the filling is completed. The pressure control valve 60 is controlled by the controller 51, and a detection signal from the filling sensor 80 is inputted into the controller 51.
The pressure control valve 60, the flow rate detecting valve 70, and the filling sensor 80 are mounted on a body 90 as shown in FIG. 8. The body 90 is provided with a pump port 91 into which pressurized oil flows from the hydraulic pump 53, an output port 92 which supplies pressure oil into the clutch chamber 54, and drain ports 93 and 94, shown in FIG. 8, which return oil returning or leaking from the clutch chamber 54 to a tank 95. The pressure control valve 60 includes a pressure control valve spool 61 (hereinafter, called a control valve spool 61), a piston 62, a proportional solenoid 63, and a first spring 64. The right end portion of the control valve spool 61 is abutted to a plunger 63a of the proportional solenoid 63, and the spring 64 is abutted to the left end portion thereof, thereby pressing the control valve spool 61 toward the proportional solenoid 63 side. The pressure in an oil passage 67 (the pressure to the clutch chamber 54) acts on an oil chamber 65 inside the spool 61, which is defined by the control valve spool 61 and the piston 62 in an oil passage 66 formed inside the control valve spool 61.
The flow rate detecting valve 70 includes a flow rate detecting valve spool 71 (hereinafter, called a detecting valve spool 71), a second spring 72, and a third spring 73. Three raised portions are formed on the detecting valve spool 71, and these raised portions divisionally form a first oil chamber 74, a second oil chamber 75, and a third oil chamber 76. A hole 77 is provided in the raised portion of the detecting valve spool 71 which is between the second oil chamber 75 and the third oil chamber 76. The detecting valve spool 71 has three different pressure receiving areas Aa, Ab, and Ac, and allows these areas to have the relationships (Aa+Ac)&gt;Ab, and Ab&gt;Ac. The second spring 72 is abutted to and inserted into the left end portion of the detecting valve spool 71, and the third spring 73 is abutted to and inserted into the right end portion thereof. When pressure is not generated in the second oil chamber 75 and the third oil chamber 76, the detecting valve spool 71 is held in a neutral position where the second spring 72 and the third spring 73 respectively have free length. Accordingly, when the detecting valve spool 71 is not operated (the illustration shows a situation in which it is operated), the pressurized oil flowing into the flow rate detecting valve 70 via the pump port 91 stays in the first oil chamber 74. In the above, the second spring 72 and the third spring 73 work as springs for returning the detecting valve spool 71, and when the pressurized oil is not supplied, the detecting valve spool 71 is designed to be in its neutral position.
The filling sensor 80 includes a sensing pin 81, a first resistance Ra and a second resistance Rb which are connected in series, and an insulator 82. The sensing pin 81 is attached at the body 90 with the insulator 82 between them in the position where the detecting spool 71 abuts to the right end portion thereof when it is moved. A lead wire 83 is extended from the sensing pin 81, and the lead wire 83 is connected to a point between the first resistance Ra and the second resistance Rb. A predetermined direct voltage is applied across the first resistance Ra and the second resistance Rb, and the body 90 is grounded by means of a ground wire 84.
Next, the operation in the aforesaid configuration will be explained. When the clutch of an applicable speed gear is engaged, the controller 51 outputs a current Ia (at point in time Ta) as a trigger command to the proportional solenoid 63 of the pressure control valve 60. Thereafter, the controller 51 lowers the command current Ia to an initial pressure command current Ip corresponding to the initial pressure valve Pp of the clutch oil pressure, and waits until the filling is finished in this state.
As a result of inputting the current Ia of the trigger command, the plunger 63a of the proportional solenoid 63 moves the control valve spool 61 in the leftward direction in the drawing. Thereby, the pressurized oil from the hydraulic pump 53 flows into the second oil chamber 75 of the detecting valve spool 71 via the pump port 91, the control valve spool 61, and the oil passage 67 (the arrow Ya shown in FIG. 8). The pressurized oil, which has flowed into the second oil chamber 75, flows into the clutch chamber 54 via the hole 77 of the detecting valve spool 71, the third oil chamber 76, and the output port 92. At this time, the hole 77 causes a differential pressure between the second oil chamber 75 and the third oil chamber 76, thereby moving the detecting valve spool 71 in the leftward direction in the drawing. As a result, the first oil chamber 74 and the second oil chamber 75, which have been closed to each other by the body 90, are now opened to each other. The pressurized oil from the hydraulic pump 53 then passes through the opened portion (Yb), merges with the oil Ya from the aforesaid control valve spool 61, and flows into the clutch chamber 54 via the hole 77 of the detecting valve spool 71, the third oil chamber 76, and the output port 92. The pressurized oil continues to flow until the clutch chamber 54 becomes full.
When the clutch chamber 54 is filled with oil, the filling is finished, the oil does not flow any more, and the differential pressure in the hole 77 between the second oil chamber 75 and the third oil chamber 76 no longer exists. Thereby, the detecting valve spool 71 is moved in the rightward direction, and is returned to its neutral position. Further, at this time, due to the relationship among the areas of the pressure receiving areas of the detecting valve spool 71, that is, (Aa+Ac)&gt;Ab, and the force to which the returning force of the second spring 72 is added, the detecting valve spool 71 is moved in the rightward direction. When the detecting valve spool 71 is returned, the pressurized oil from the hydraulic pump 53 fills the clutch chamber 54, and causes chute pressure. Further, the detecting valve spool 71 is moved in the rightward direction, thereby bringing the detecting valve spool 71 into contact with the sensing pin 81. By this contact, it is detected that the clutch chamber 54 is filled with oil, and that the filling is finished.
As described above, in the aforesaid art, at a point in time when the clutch chamber 54 is completely filled, a peak pressure occurs as a result of a delay in the response of the pressure control valve 60 and the flow rate detecting valve 70. The peak pressure doesn't matter in a higher throttle area at higher engine speed. However, a disadvantage in that a shock to the vehicle or noise, to thereby make the operator feel uncomfortable, occurs especially in a low throttle area at lower engine speed in a construction vehicle which has tires, such as a wheel loader, which requires smooth engagement of the clutch in both the forward movement and the reverse movement. In order to eliminate the disadvantage, it is suitable to reduce the flow rate of the pressurized oil supplied to a clutch chamber 8 (see FIG. 1) before the clutch chamber 8 proposed by the present invention is filled with the pressurized oil, thereby achieving a smooth engagement of the clutch. However, unless the engagement of the clutch is achieved in a certain period of time, a disadvantage occurs, thereby giving the operator an uncomfortable feeling that a slow start is being made. When a control is to be carried out to smoothly engage the clutch in a certain period of time for the aforesaid reason, the following difficulties occur. Specifically, the first one is that the volume of the clutch chamber is not fixed, but is "varied" according to manufacturing errors of the components, such as a clutch disc, a clutch piston, or a piston case. The second one is that the clutch disc can be worn, thereby varying the volume of the clutch chamber.
FIG. 10 is a graph for explaining the above description, and shows a situation in which the engine speed is in a lower range. The elapsed time T needed to fill the clutch chamber 54, is plotted in the axis of abscissa, and the volume V of the clutch chamber 54 is plotted in the axis of ordinates. In the graph, the line L represents the pressurized oil supply to the clutch chamber 54, the supply flow rate is larger along the line LA extending from the engaging period in time 0 to the period in time Ta, and a smaller supply flow rate is shown by the solid line LB extending from the point in time Ta. The gradient of the solid line LB from the period in time Ta shows the increase ratio of the supply rate of the pressurized oil which does not cause a shock or a noise in the vehicle. The portion between the points in time Tb and Tc shows an allowable engaging time interval when the engine speed is in a lower range. The portion between the values Vb and Vc for the volume of the clutch chamber 54 shows variations in the volume of the clutch chamber 54. The clutch is assembled with the volume Vd which is between the variations Vb and Vc of the volume of the clutch chamber 54. It is difficult to match a point of completing the engagement of the clutch, that is, a completion point Ps of the solid line LB, to the volume Vd as well as to the allowable engaging time interval Tb to Tc. Specifically, it is followed by a difficulty to place the completion point Ps within the range between the points PA and PB on the solid line LC. In addition, even if the point PS temporally falls in a right place, when the clutch disc is worn out, the engaging time goes out of the allowable engaging range between the points in time Tb and Tc, which becomes a disadvantage .